The invention relates to a pump for liquid chromatograph, and in particular to a pump for liquid chromatograph, which is suitable for feeding liquid at a low flow rate.
In general, a conventional pump for liquid chromatograph has a plunger which reciprocates in a cylinder so as to suck and discharge eluent. In order to feed a trace of solution with a high degree of accuracy, it is required to decrease the diameter or stroke of the plunger, the smaller the solution to be fed, the more difficult the discharge of air bubbles stagnating in the inside of the pump or a solution feed passage.
For example, as disclosed in JP-A-2000-39427, there has been well-known a pump system which can feed a solution with no influence of air bubbles in the solution, and which comprises two pump units each including a first and a second pump head having different capacities, and driven by a different diameter plunger with a large diameter part and a small diameter part coaxially formed, and in which the two pump heads driven by the large diameter parts of the two different diameter plungers are used as a first pump while the two pump heads driven by the small diameter parts of the two different plungers are used as a second pump, and these first and second pumps are connected in series. Further, a heater and a gas/liquid separator are connectred to the discharge port of the first pump, and accordingly, after the air bubbles are removed from the eluent by this gas/liquid separator, the eluent is sucked into the second pump for feeding the eluent.
The above-mentioned prior art is adapted to feed a solution with a high degree of accuracy without influence of air bubbles in the solution, but there has not been sufficiently taken for this prior art such a consideration that a trace of eluent is fed with a high degree of accuracy. Further, in this prior art, the flowing velocity of eluent from the first pump which is driven by the large diameter parts of the different diameter plungers is set to be higher than that of eluent from the second pump driven by the small diameter parts of the different diameter plungers (preferably higher by two folds), and accordingly, a minimum discharge quantity of this pump system is determined by the discharge volume of the second pump. Thus, in order to set a minimum discharge volume of the pump system to a trace value, it is required to reduce the outer diameter of the second plunger which determines the discharge volume. However, it is difficult in view of machining accuracy and strength to decrease the diameter of the plunger for setting the minimum discharge quantity to a trace value.
Further, in the conventional technology, should the small diameter part of the plunger be decreased in its diameter while the large diameter thereof is unchanged so as to decrease the discharge volume of the second pump, the discharge volume of the first pump would relatively be increased. Meanwhile, even though the small diameter part of the plunger is increased in its diameter so that the discharge volume of the first pump is decreased but the discharge volume of the second pump is increased. Thus, there has been presented such a disadvantage that it is difficult to decrease the liquid feed quantity to a trace value with the use of the different diameter plunger.
Should the discharge volume of a pump be decreased so as to set the liquid feed quantity to a trace value, there would be caused such a disadvantage that a long time is required for priming a solution into several passages and the like, and for discharging air bubbles therefrom upon a start of a test.